144 ETHEL NICHOLSON BROWNE 



SO that all stages in the spermatogenesis are to be found through- 

 out the summer and fall; in the 13-species, on the other hand, 

 the evokition of stages is very slow, so that at one time the testis 

 is filled with cells in the growth stages, then with dividing cells, 

 and in the late summer only spermatozoa are to be found. Prob- 

 ably as a result of this, the cells of the former species are smaller 

 than the latter. Whether these two characters are really corre- 

 lated with the chromosome number, or whether it is a mere coin- 

 cidence can probably be determined by work on other species. 



Just what the status of each species is in relation to the others 

 cannot be told at present. I think there is little doubt that N. 

 indica is a species derived from N. undulata, as both the somatic 

 characters and the chromosomes indicate this. N. shooterii is 

 particularly puzzling because it seems primitive with regard to 

 the XY pair and yet it seems to represent a transition stage with 

 regard to the autosomes. 



IX. CONSTANCY OF THE CHROMOSOMES 



Notonecta adds to the ever increasing mass of evidence that 

 the chromosomes are definite bodies with individual peculiarities 

 and characteristics. Each species is characterized by some spe- 

 cial behavior or structure or position of certain of the chromo- 

 somes, and this characteristic is constant for all the individuals 

 and all the cells of that species with few exceptions. I would 

 especially emphasize the constancy in the arrangement of the 

 chromosomes. There is always some particular grouping of the 

 chromosomes, different in the two divisions, but characteristic 

 of some particular species. 



X. SUMMARY 



1) An XY pair of chromosomes is present in all six species, 

 the components dividing separately in the first division and go- 

 ing to opposite poles in the second. The components frequently 

 lie side by side in the second metaphase. 



2) The X and Y chromosomes are almost equal in most cells 

 of certain individuals of N. shooterii. They are more nearly 



